Titania (titanium dioxide, TiO.sub.2) is a material characterized by many interesting and useful chemical and physical properties. Because of this, titania and titania-coated surfaces are employed in a wide range of commercial and industrial applications including the catalysis and photo-catalysis of chemical reactions, pigmentation, the filtering or reflection of electromagnetic radiation, modification of the electrical properties of surfaces, chemical resistance and absorbency, and so forth. Vast quantities of pure titania and of titania-coated materials are employed around the world in such applications. The great commercial importance of titania and titania-modified materials provides the motivation behind research and development efforts directed towards new and improved techniques for the creation and processing of such materials.
In addition, zinc sulfide-based phosphors (typically doped with copper) may be stimulated to emit visible light either by the absorption of ultraviolet radiation (as in a fluorescent lamp) or by the use of electrical energy in a so-called electroluminescent lamp, where a layer of the phosphor is sandwiched between a front transparent electrode and a back non-transparent electrode, with a layer of insulating material (typically barium titanate) sandwiched between the phosphor layer and the back electrode. In both cases, however, the luminescent efficiency of the phosphor degrades much more rapidly if the phosphor is exposed to a moisture-containing atmosphere than if it is exposed only to a very dry atmosphere (while being stimulated by ultraviolet radiation or by the application of an alternating electromagnetic field). The formation of a thin continuous and conformal coating of titania on the surfaces of zinc sulfide phosphor particles protects the particles from the effects of atmospheric moisture.
Continuous and conformal titania films may be formed by a variety of processes including the oxidation of evaporated titanium films, the reactive sputtering of titanium in an oxygen-containing atmosphere, the anodic oxidation of a titanium surface, and by the oxidation of a gaseous titanium compound such as titanium tetrachloride or titanium isopropoxide (Ti[(CH.sub.3).sub.2 CHO].sub.4). The gas-phase oxidation of a titanium compound to form a titanium oxide film is an example of chemical vapor deposition (CVD). Such reactive CVD processes with inorganic or organometallic titanium precursors typically require relatively high temperatures (in the vicinity of 400.degree. C. or above) and they typically require a source of gaseous oxygen, in addition to the gaseous titanium-containing precursor.
The requirements for both relatively high reaction temperatures and oxidizing atmospheres preclude the use of such CVD techniques to form titania coatings upon the surfaces of materials which cannot withstand being heated to such temperatures in an oxidizing atmosphere. This constraint is particularly significant if titania coatings are to be formed upon the surfaces of reactive finely divided materials since, in this case, the high surface-to-volume ratio may render the material extremely vulnerable to attack under such conditions. On the other hand, oxide coatings cannot be formed efficiently upon the surfaces of particles of a finely divided material by other than a CVD technique with the powdered material suspended in a gas-fluidized bed.
The disadvantages of the existing methods for the formation of titanium oxide coatings upon the surfaces of finely divided materials are completely eliminated by the method described in this disclosure. Specifically, the disclosed invention provides a method for the formation of continuous and conformal titania coatings of any desired thickness upon the surfaces of finely divided materials without exposing the powdered material to an oxidizing atmosphere and without exposing the material to temperatures in excess of 300.degree. C.